The present invention relates to a start up circuit for commutation power supplies (PWM) or DC/DC converters and to a commutation type power supply comprising such a start up circuit.
Most of the integrated control circuits for commutation power supplies require for their starting a current to charge a feeding capacitor of the integrated circuit. Such a current comes from the start up circuit (start-up), constituted, in the simplest case, by a resistance connected to the feeding line of the power circuits of the power supply.
The integrated control circuit activates when the voltage on the capacitor reaches a prefixed value equal to a minimum working threshold voltage of the integrated circuit called start-up voltage. The current supplied by the start up circuit must be greater than the feeding current of the integrated circuit, called start-up current, otherwise the capacitor cannot be charged and the power supply will not be activated. Even, the supplied current must be sufficiently greater than the start-up current in order to charge the capacitor in reasonably brief times (i.e. in less than 1 s).
Besides, in the case in which the start up circuit is realized by means of a resistance, it remains active also when the converter is working and therefore it dissipates power. Particularly in the so-called xe2x80x9cuniversalxe2x80x9d power supplies, that is, able to work with both the American power net (110 VAC) and the European one (230 VAC), in a start up circuit able to supply a sufficiently elevated current when fed with 110 VAC, in the case of 230 VAC feeding the current will be proportionally more elevated and the related dissipation will increase in a quadratic way. Since lately there are more and more restrictive requests about the consumption reduction of the power supplies, the start up circuit contribution to the total consumption can be a problem.
Once that the control integrated circuit is activated, its consumption increases greatly, overcoming the current supplied by the start up circuit. The voltage at the capacitor terminals will start decreasing, but at this point the power supply begins to work and it will send a self feeding voltage (take-over voltage) to the control integrated circuit. Then the capacitor supports the working of the control circuit up to reaching the power supply steady state voltage value. The self feeding voltage is generally obtained from an additional winding performed on the power supply transformer to which an opportune rectification and filtration circuit is connected.
The start up circuit has a very important role also in short circuit conditions. The disappearance or the diminution of the self feeding voltage, in response to a short circuit and therefore to the voltage at the terminals of the capacitor under the minimum working threshold of the controller, generally called minimum working voltage, will cause the momentary stop of the converter working. Once it is off, the capacitor will again be charged by the start up circuit up to the reaching of the start-up voltage, and it will reactivate the power supply. If the short circuit condition is still present the previous steps will repeat. The time between two following reactivations of the control circuit depends on the current that the start-up circuit supplies and on the capacitor value. This intermittent working, called hiccup mode, allows, in short circuit, to have in the power supply an average current value lower than those relative to a continuous working in the same conditions, in this way decreasing the dissipation of the power supply circuits that could otherwise be destroyed. It is therefore clear that a low intermittent frequency and accordingly a low starting current is essential for the protection of the converter in case of breakdown.
The start up circuit must therefore have the characteristics of: charging the capacitor quickly and therefore it must supply a relatively high current; decreasing and/or eliminating its consumption once that the power supply is working, and therefore it must supply a relatively low current or nothing; getting an intermittent frequency as lower as possible in short circuit conditions and therefore it must supply a relatively low current.
A system used in the construction of the start up circuits is to charge the capacitor by means of a resistance connected to the feeding line of the supply power devices or connected in series to a diode and applied directly to the AC power net.
The starting time is related to the time constant introduced by the resistance and by the capacitor. The capacitor must be determined in order to be able to supply the control integrated circuit working during the phase in which the voltage generated by the self feeding circuit has not been activated yet. An elevated resistance does not allow a quick starting of the circuit. A low resistance starts the system quickly but penalises the two other cited requirements severely. As already said, this problem is as more remarkable as wider the input voltage range of the converter is. In fact it will be difficult, if not impossible, to find a resistance value that is a good compromise between a starting of the circuit in brief times and a low dissipation.
Another system used in the realization of the start up circuits is to charge the capacitor by means of a low value resistance placed in series with a controlled switch that at the activation of the self feeding voltage is opened. In this way we have a quick charging time, we have not consumption (or however it is reduced) during the normal working of the converter, but we have a high intermittent frequency in case of short circuit.
Another system used in the realization of the start up circuits is that used by the integrated circuit LR645 marketed by the Supertex inc. firm which is a linear voltage regulator, able to charge the capacitor quickly by bringing the voltage at its terminals to a value superior to the start-up voltage of the control integrated circuit. The device is turned off when the self feeding voltage overcomes the voltage supplied by LR645. If the voltage generated by the self feeding circuit decreases in an appreciable way, for instance in low load conditions of the power supply, and it should drop down under the voltage supplied by the LR645, the device would be turned on with consequent elevated power dissipation.
Another system used in the construction of the start up circuit is that used by the integrated circuit LR745 commercialised by the Supertex inc. firm. Such a device charges the capacitor with a prefixed current. When the voltage on the capacitor, during the decreasing due to the activation of the control circuit, goes below a threshold, the device is turned off. To turn it on, the voltage on the capacitor must go below a value inferior to the minimum working voltage of the control integrated circuit. In this way the risk of turning on with a low load is eliminated. In case of short circuit we have only a small increase of the circuit starting time, that is the time necessary for the capacitor voltage to go under the minimum working voltage of the integrated control circuit, but it is not able to assure a sufficiently long time between two following activations of the converter.
In view of the state of the art described, it is an object of the present invention to get a start up circuit for commutation power supplies able to supply a sufficiently elevated current in the phase of starting of the control circuit.
Another object is to increase the time occurring between two following activations during the short circuit of the commutation power supply.
A further object is to reduce the power dissipation of the start up circuit when the commutation power supply is working.
According to the present invention, these and other objects are attained by means of a start up circuit for commutation power supplies having an input terminal and an output terminal comprising: a first current generator able to supply a first prefixed current connected between said input terminal and a first node; said first node is coupled to said output terminal; characterized by further comprising a second current generator able to supply a second prefixed current connected between said second input terminal and a first terminal of a first controlled switch; the value of said second prefixed current is greater than or equal to the value of said first prefixed current; a second terminal of said first controlled switch is connected to said first node; a control circuit of said first controlled switch able to maintain closed said first controlled switch in the case in which the voltage on said output terminal is lower than a first prefixed voltage.
Such objects are also attained by a commutation power supply comprising a control circuit with amplitude modulation of the pulses; a start up circuit of said control circuit with amplitude modulation of the pulses according to claim 1.